Anomalous spin dynamics in CdCu2(BO3)2 revealed by 11 B NMR and ZF- μsR

Abstract

We report 11Bnuclear magnetic resonance (NMR)and zero-field muon spin relaxation (ZF-μSR)measurements of the anisotropic spin tetramer system CdCu2 (BO3) 2, which consists of strongly interacting Cu(1) dimers and weakly coupled nonfrustrated Cu(2) spins. Long-range magnetic order is observed at TN = 9.8 K by a critical divergence of the nuclear spin-lattice relaxation rate 1/T1 and an appearance of well-defined muon-spin precessions. In addition, we find the spectroscopic signature for a magnetic anomaly at T ∗ = 6.5 K, a few kelvins below TN. For temperatures below T ∗, 1/T1 ceases to follow a T 4.9(1) behavior. Instead, upon cooling below T ∗, both 1/T1 and 1/T2 become temperature-independent with a subsequent small drop. The muon frequency and the relative fraction of the Cu magnetic sites show anomalies mostly for Cu(2) spins at T ∗. This is ascribed to a Cu(2) spin reorientation. Site-specific change in magnetic structure is discussed in terms of the energy hierarchy of involved exchange interactions and an additional intertetramer interaction that controls magnetism at temperature T ∗.